Re: US add on required?

Yes, even if your base plan includes US talk and text, it's only for sending to/receiving from the US while you are IN Canada. You will need to purchase a roaming add-on for when you travel to the US, or else get a local SIM where you are going. PM's US roaming add-ons only last for 10 days and they start right from the moment they are purchased.

Re: US add on required?

When I travel to the US it's usually for 3 weeks at a time. I purchase a local plan with unlimited text, data, and talk. I find it's just much more convenient, and quite cheap. On our Christmas vacation I purchased a one month plan: 4 phones for $120 US.